Lithium secondary batteries are an important source of portable DC power and, despite recent developments in fuel cells, are likely to remain so for many years. Portable electronic and electrical systems increasingly require high-power, compact, durable power sources for effective performance and thus, improved portable power sources are required. Of particular interest are high-energy lithium-ion batteries that can be recharged in a fraction of the time required for existing systems and also have improved battery cycle life.
One way to achieve the desired increase in performance is to use an improved active cathode material. It is known to use lithium transition metal oxides as high performance active cathode materials for secondary lithium batteries and lithium cobalt oxide is one of the most commonly used commercial materials. Another material of interest is lithium iron phosphate, but lithium iron phosphate synthesised in bulk has a high electrical resistance. This is undesirable for high power applications and necessitates incorporation of carbon into the electrode material to increase its conductivity and hence, rate capability.
Battery electrodes are typically fabricated by coating a paste comprising the active cathode material, binder and conductive carbon onto a current collector such as aluminium foil. The active material is the main component of the electrode, providing the required electrochemical capacity. The binder provides inter-particle connectivity and adherence to the current collector, while the conductive carbon boosts the level of electronic conductivity for the electrode reaction. The size of the active material particles is crucial for fabrication of uniform and adherent coatings, as well as for the desired level of electrode performance, and it is well known that large particles not only limit the thickness of the electrode to high values, but also result in uneven coatings. With regard to electrode performance, large particles and thick coatings limit the rate at which lithium ions can diffuse in and out of the electrode during charge and discharge and hence, limit the power/rate capability of the electrode.
During the charge and discharge of lithium ion batteries, lithium ions move in and out of the active cathode material particles causing them to expand and contract in the process. The repeated expansion and contraction of particles with continuous cycling gradually results in loss of electrical contact between particles and a consequent increase in the resistance of the electrode. In turn, the increase in resistance results in poor utilisation of the electrode because charge and discharge cut-off voltages are attained prematurely before all the capacity can be extracted. This is a phenomenon known as ‘capacity fade’.
Thus, for batteries to exhibit high rate, long cycle life and good utilisation, it is desirable that battery electrode materials are composed of small particles having a high porosity. This results in a large active surface area for the electrode and enables very thin electrode coatings to be fabricated. High surface area (measured as specific surface area) also promotes fast electrode reactions, thereby enhancing rate performance and power capability.
Traditional methods for achieving high surface area battery electrode materials include ball milling to achieve small particle size, and pyrolysis or solid state routes to obtain porous materials. These methods tend to suffer from a number of disadvantages, however, including agglomeration of smaller particles, expense or being unsuitable for scale-up. Compatibility with existing manufacturing processes for battery electrodes can also be a problem.
Alternatively, lithium transition metal oxides can be synthesised directly as nanoparticles. One known method of producing nanoparticles of a lithium transition metal oxide, for example, is by direct solution precipitation of a single phase metal hydroxide or a multi-phase mixed metal hydroxide, followed by a calcining step which converts the hydroxide or hydroxides to a respective oxide or compound oxide product. Another known method of producing nanoparticles of lithium transition metal oxides is by solution sol-gel synthesis and, by way of example, Hahn et al (Proc. 9th Asia Pacific Physics Conference, Hanoi, Vietnam, Oct. 25-31, 2004) describes the reaction of lithium nitrate with cobalt nitrate in the presence of a citric acid reaction agent to produce lithium cobalt oxide powder having a particle size around 100 nm. Lithium cobalt oxide nanomaterials produced by the aforementioned methods, however, tend to have an upper achievable surface area limit of about 5 m2/g.